Which factor deficiency would most likely result in prolonged APTT and normal PT?

The scenario described—a prolonged Activated Partial Thromboplastin Time (APTT) and a normal Prothrombin Time (PT)—is indicative of a deficiency in one of the intrinsic pathway factors of coagulation. Factor VIII is a critical component of this pathway and its deficiency primarily affects the APTT.

When Factor VIII is deficient, the intrinsic pathway does not function properly, leading to prolonged APTT. The normal PT indicates that the extrinsic pathway, which involves Factor VII, is functioning appropriately. This is consistent with the understanding that a deficiency in Factor VII would lead to a prolonged PT rather than affecting the APTT. Similarly, deficiencies in Factor V or Factor X would result in coagulopathy affecting both pathways, leading to prolonged PT and APTT since they are central components of the coagulation cascade.

Thus, Factor VIII deficiency is specific for causing the observed changes, making it the most likely cause of the condition described in the question.